Photodegradation of RDX in aqueous solution:: A mechanistic probe for biodegradation with Rhodococcus sp.

Recently we demonstrated that Rhodococcus sp. strain DN22 degraded hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) (1) aerobically via initial denitration followed by ring cleavage. Using UL (14)C-[RDX] and ring labeled (15)N-[RDX] approximately 30% of the energetic chemical mineralized (one C atom) and 64% converted to a dead end product that was tentatively identified as 4-nitro-2,4-diaza-butanal (OHCHNCH(2)NHNO(2)). To have further insight into the role of initial denitration on RDX decomposition, we photolyzed the energetic chemical at 350 nm and pH 5.5 and monitored the reaction using a combination of analytical techniques. GC/ MS-PCI showed a product with a [M + H] at 176 Da matching a molecular formula of C(3)H(5)N(5)O(4) that was tentatively identified as the initially denitrated RDX product pentahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene (II). LC/MS (ES-) showed that the removal of RDX was accompanied by the formation of two other key products, each showing the same [M - H] at 192 Da matching a molecular formula Of C(3)H(7)N(5)O(5). The two products were tentatively identified as the carbinol (III) of the enamine (II) and its ring cleavage product O(2)NNHCH(2)NNO(2)CH(2)NHCHO (IV). Interestingly, the removal of III and IV was accompanied by the formation and accumulation of OHCHNCH(2)NHNO(2) that we detected with strain DN22. At the end of the experiment, which lasted 16 h, we detected the following products HCHO, HCOOH, NH(2)CHO, N(2)O, NO(2)(-), and NO(3)(-). Most were also detected during RDX incubation with strain DN22, Finally, we were unable to detect any of RDX nitroso products during both photolysis and incubation with the aerobic bacteria emphasizing that initial denitration in both cases was responsible for ring cleavage and subsequent decomposition in water.